As an apparatus for amplifying signal light in optical communications, an optical fiber amplifier has been known. As disclosed in each of Japanese Patent Application Laid-Open No. HEI 10-56227 and Japanese Patent Application Laid-Open No. HEI 10-90547, such an optical fiber amplifier is configured such that signal light is propagated through an amplifying optical fiber doped with a rare-earth element; and pumping light, having a predetermined wavelength different from that of the signal light and causing the doped rare earth to release light having a wavelength identical to that of the signal light, is input into the amplifying optical fiber from both ends thereof, so as to amplify the signal light.
Semiconductor light-emitting devices such as light-emitting diode (LED) and laser diode (LD) are in common use for the pumping light source. The light emitted from one such semiconductor light-emitting devices is guided to the amplifying optical fiber by a waveguide optical fiber.
In general, the output face of the light-emitting region (active region) of a semiconductor light-emitting device has a rectangular form, and the length L of the longer side of the output face of the light-emitting region is longer as its optical output power is greater. As the semiconductor light-emitting device favorably usable for the pumping light source, those having an L of 100 .mu.m or 50 .mu.m and the like have been known. In order to favorably guide the light emitted from such a kind of semiconductor light-emitting device to a waveguide optical fiber, while facilitating the positional adjustment between the semiconductor light-emitting device and the optical fiber, it has conventionally been a common practice to make the core diameter D of the waveguide optical fiber not shorter than the length L of the longer side of the output face of the light-emitting region of the semiconductor light-emitting device (see catalogs of LD modules MLD100-FC and MLD300-FC manufactured by Mitsui Chemicals, Inc., catalog of LD module AOC980-120-HHL200 manufactured by Applied Optics Corp., and catalog of LD module SDLO-4000 manufactured by SDL, Inc., for example).